Tunable filters can provide more than one filter response using tuning elements integrated into the filter topology and therefore a single tunable filter can be adapted to multiple frequency bands. The most common tunable parameter in microwave filters is the center frequency. In discrete tuning, finite values of center frequencies are supported by the tuning devices whereas in continuous tuning a continuum range of frequencies is supported. Different tuning methods including mechanical, magnetic, and electrical tuning have been proposed to construct tunable filters with either discrete or continuous tuning. Tuning methods are evaluated in terms of different metrics including their tuning range, quality factor, power handling capability, fabrication complexity, size, and tuning speed. None of the tuning methods is superior in terms of all metrics and, therefore, depending on the application and design constraints, one tuning method might be preferred over the other in a particular application or use.
Mechanical tuning is easy to implement and provide a relatively high power handling capability. However, they have low tuning speed and are usually bulky in size. Micro Electro-Mechanical Systems (MEMS) technology is an attractive approach to realize tunable filters with a compact size. Integration of MEMS devices with three dimensional resonators is a challenge leading to a significant degradation in the resonator Q-factor.
Accordingly, there remains a need in the industry for an improved tunable filter.